Recently, there have been proposed technologies for improving sleep of a human subject by detecting biorhythms of the human subject, such as body motion, respiration and heartbeat, and generating a sound in accordance with the detected biorhythms (for example, see Japanese Patent Application Laid-Open Publication No. H4-269972). In addition, there have also been proposed technologies for adjusting, in accordance with a relaxation state of a human subject, at least one of a type, a loudness, and a tempo of a sound generated (for example, see Japanese Patent Application Laid-Open Publication No. 2004-344284).
During sleep, rapid eye movement (REM) sleep, which is characterized by light sleep, and non-REM sleep, which is characterized by deep sleep, alternate in cycles of approximately 90 minutes. However, differences in cycles of REM sleep and non-REM sleep exist between individuals. Moreover, a surrounding environment, a period between sleep onset and awakening, as well as time changes in sleep depths may not always be the same for a single human subject.